Microelectronic or optoelectronic package having a polybenzoxazine-based film as an underfill material

ABSTRACT

Microelectronic and optoelectronic packaging embodiments are described with underfill materials including polybenzoxazine, having the general formula:

BACKGROUND OF THE INVENTION

1). Field of the Invention

This invention relates to microelectronic and optoelectronic packagesand, more specifically, to an underfill material in such packages.

2). Discussion of Related Art

A microelectronic or an optoelectronic package usually includes either amicroelectronic semiconductor die or an optoelectronic die mounted to apackage substrate. The package substrate provides rigidity to theoverall package, and often also provides electronic interconnect betweenthe die and another device. An underfill material is introduced betweenthe die and the package substrate.

The semiconductor die, for example, may have conductive members on alower surface thereof, and are positioned on top of an upper surface ofthe package substrate. The conductive members provide electriccommunication between the semiconductor die and the package substrate.The conductive members also mount the conductor die to the packagesubstrate. The conductive members are, however, relatively fragile.Differences in coefficients of thermal expansion (CTE) between thesemiconductor die and the package substrate cause relative shrinkage orexpansion between the semiconductor die and the package substrate. Theunderfill material serves to reduce stresses on the conductive membersdue to such relative expansion or contraction.

A filler material is usually included in the underfill material. Fillerloading is usually required to reduce shrinkage during curing, tocontrol CTE, and to control moisture uptake. Filler loading tremendouslyincreases the viscosity of the film at bonding temperature. Theincreased viscosity tremendously increases the bonding force per bumpand reduces the wetting properties of melted film to all contactsurfaces. The increase of bonding force per bump often cracks the chipduring chip placement, and poor wetting properties of melted filmadversely affect the joint integrity and introduces voids that shortenthe joint fatigue life.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of examples with reference to theaccompanying drawings, wherein:

FIGS. 1 a-e are cross-sectional side views of polybenzoxazine-basedfilms according to different embodiments of the invention; and

FIGS. 2 a-c are cross-sectional side views of electronic assemblieshaving polybenzoxazine-based underfill materials, according to differentembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Microelectronic and optoelectronic packaging embodiments are describedwith underfill materials including polybenzoxazine, having the generalformula:

wherein n can be any integer from 1 to 20 and is preferably an integerfrom 1 to 4 and is most preferably 1 or 2, and R₁ can be hydrogen, oneor more groups selected from hydroxyls, one or more linear or branchedalkyls of 1 to 80 and more preferably 1 to 10 carbon atoms, aromatics,alkyl substituted aromatics, aromatic substituted alkyls of 6 or 20carbon atoms, mono or poly fluorine substituted alkyls of 1 to 20 carbonatoms, a mono or poly fluorine substituted compound having at least 1aromatic ring and 6 to 20 carbon atoms, and phenolic compounds of 6 to20 carbon atoms (for the purpose of this specification phenoliccompounds may have more than one hydroxyl group as defined in chemicaldictionaries) including poly phenolic compounds having on average 6 to20 carbon atoms per phenol group. When n is 2, 3 or 4, R₁ can beselected from the connecting groups below:

Depending on whether phenolic or phenoxy repeat units are desired in thepolybenzoxazine, it may be desirable that R₁ be ortho, meta, or para tothe oxygen atom of the benzoxazine monomer of Formula A. Further, R₂ maybe an alkyl of 1 to 40 carbon atoms, an aromatic, an alkyl substitutedaromatic or aromatic substituted alkyl of 6 to 40 carbon atoms, mono orpoly fluorine substituted alkyl of 1 to 20 carbon atoms, a mono or polyfluorine substituted compound having at least one aromatic ring and 6 to20 carbon atoms or an amine of 1 to 40 carbon atoms including polyaminesand aromatic, alkyl substituted aromatic, or aromatics substitutedalkyls having 6 to 40 carbon atoms. Furthermore, each benzene ring, asshown by (R₃), where p is an integer from 0 to 3 and R₃ is as definedlater, can have more than one substituent of the same structure or amixture of the R₃ structures.

The variable m can be an integer from 0 to 5 and R₃ can be H or R₂.Preferably R₃ is not the amine or polyamine components of R₂. PreferablyR₃ is an alkyl of 1 to 9 carbon atoms such as CH₃, C₂H₅, C₃H₇, or C₄H₉,or a mono or poly fluorinated alkyl of 1 to 9 carbon atoms such as CF₃,C₂F₅, or C₃F₇. These R₁ compounds are well known to those familiar withphenolic compounds. Generally R₁ can be any of the known connectinggroups than interconnect two or more phenols. Known connecting groupsrefers to those which are present in commercially available phenols, arein experimentally available phenols, and phenols whose synthesis aredescribed in the published literature. Examples of such phenols include:

As the cationically initiated ring opening polymerization ofmonofunctional benzoxazines generally results in linear polymers, it isdesirable that at least 25 mole percent, more desirably at least 50 molepercent and preferably at least 75 mole percent or 90 mole percent ofthe R₁ groups are not an additional hydroxyl groups or a phenolic orpolyphenolic compound and n in Formula A is 1. Also desirably at least25 mole percent, more desirably at least 50 percent, still moredesirably at least 75 or 90 mole percent of the R₂ groups are neitherpolyamines nor include additional benzoxazine compounds. Theselimitations are desirable for thermoplastic polybenzoxazines, but it isunderstood that if thermoset polybenzoxazines are desired, the amount ofdifunctional or polyfunctional benzoxazine monomers (those where n is 2to 20 or R₂ is a polyamine or includes additional benzoxazine rings)could be higher. When high molecular weight thermoplasticpolybenzoxazines are desired, desirably the number average molecularweight of the polybenzoxazine is at least 5,000 and more desirably atleast 10,000.

As is well known, benzoxazine monomers are made from the reaction ofthree reactants, aldehydes, phenols, and primary amines by proceduresusing a solvent or known as solventless systems. U.S. Pat. No. 5,543,516sets forth a generally solventless method of forming benzoxazinemonomers. An article by Ning and Ishida in Journal of Polymer Science,Chemistry Edition, vol. 32, page 1121 (1994) sets forth a procedureusing a solvent which can be used to prepare benzoxazine monomers. Theprocedure using solvents is generally common to the literature ofbenzoxazine monomers.

The preferred phenolic compounds are phenol or cresol, but can includediphenols (e.g., bisphenol-A), triphenols, etc., e.g., polyphenols,wherein each phenolic group in the phenolic compound has on averageabout 6 to about 20 carbon atoms per phenol group. The use of phenolswith two or more hydroxyl groups reactive in forming benzoxazines mayresult in branched and/or crosslinked products. The groups connectingthe phenolic groups into a phenol can be branch points or connectinggroups in the polybenzoxazine.

The aldehydes used to form the benzoxazine can be any aldehyde, butpreferably the aldehydes are those having from about 1 to about 10carbon atoms, with formaldehyde being highly preferred. The amine usedto form the benzoxazine can be an aromatic amine, an aliphatic amine, analkyl substituted aromatic, or an aromatic substituted alkyl amine. Theamine can also be a polyamine, although the use of polyamines will,under some circumstances, yield polyfunctional benzoxazine monomers.Polyfunctional benzoxazine monomers are more likely to result inbranched and/or crosslinked polybenzoxazines than monofunctionalbenzoxazines, which would be anticipated to yield thermoplasticpolybenzoxazines.

The amines generally have from about 1 to about 40 carbon atoms unlessthey include aromatic rings, and then they may have from about 6 toabout 40 carbon atoms. The amine of di or polyfunctional may also serveas a branch point to connect one polybenzoxazine to another.

In the past, thermal polymerization has been the preferred method forpolymerizing benzoxazine monomers. The temperature to induce thermalpolymerization is typically varied from about 150 to about 300° C. Thepolymerization is typically done in bulk, but could be done fromsolution or otherwise. Catalysts, such as carboxylic acids, have beenknown to slightly lower the polymerization temperature or accelerate thepolymerization rate at the same temperature.

Cationic polymerization initiators described in this invention have beenfound to result in polymerization of benzoxazine monomers attemperatures as low as cryogenic temperatures. Preferred temperaturesare from about −100° C. to about 250° C., and most preferably betweenabout −60 and 150° C. for ease of handling the reactants and products.Some of the cationic initiators, e.g., PCl₅, form repeating units fromthe benzoxazine monomers, that include a salt of the amine. Theserepeating units have better solubility in polar solvents, e.g., water,than similar repeating units without the amine salt. The initiators ofthe current invention can be used either in the benzoxazine melt or inthe presence of solvent, allowing the solvent content to be from 0% tonearly 100%. Many solvents can be used in cationic polymerizations, andtheir selection is known by those skilled in the art of cationicpolymerization.

The polymers from the cationically initiated polymerization ofbenzoxazine are useful as molded articles with good thermal stabilityand/or flame resistance, such as molded circuit boards, flame resistantlaminates, or other molded articles, and is a source of precursor tohigh temperature resistant chars. The common uses for high temperatureresistant chars include aircraft brake discs, equipment for sinteringreactions, and heat shields or heat shielding material. The polymerswhich include repeating units having amine salts can be used inapplications for partially or fully water soluble polymers such asviscosity control agents.

Generally, given optimal reaction conditions, cationic initiators canpolymerize benzoxazine monomers or oligomers. These include H₂SO₄,HClO₄, BF₃, AlCl₃, t-BuCl/Et₂AlCl, Cl₂/BCl₃, AlBr₃, AlBr3.TiCl₄, I₂,SnCl₄, WCl₆, AlEt₂Cl, PF₅, VCl₄, AlEtCl₂, and BF₃Et₂O. Preferredinitiators include PCl₅, PCl₃, POCl₃, TiCl₅, SbCl₅, (C₆H₅)₃C⁺(SbCl₆)⁻,or metallophorphyrin compounds such as aluminum phthalocyanine chloride,which are all known to result in similar polymers from cationicallyinitiated polymerization of unsaturated monomers. Methyl iodide (acovalent initiator), butyl lithium (an ionic initiator), and benzoylperoxide (a radical initiator) were not effective at polymerizingexperimental benzoxazine monomers.

Methyl tosylate, methyl triflate, and triflic acid appear tocationically polymerize the experimental benzoxazine monomers, althoughthe polymers did not precipitate from solution. Typically, eachinitiator initiates a polymer with from about 3 to about 1,000 to 3,000repeat units, so the amount of initiator needed on a mole percent basisrelative to the monomer is small. However, an additional initiator isneeded to compensate for loss due to adventitious moisture and otherreactants that deactivate cations, that may be present in the monomerssolvents, etc. Desirably about 0.001 to about 50 mole percent initiatorbased upon the monomer and more desirably from about 0.01 to about 10mole percent initiator is used for these cationically initiatedpolymerizations.

EXAMPLES

Several factors have been found to change the structure of thebenzoxazine polymers prepared by cationic polymerization. These factorsinclude polymerization temperature, the particular cationic initiator,and competing reactivity between the ortho-carbon of the benzene ringand the basic nitrogen atom of the oxazine ring.

For example, the following results were obtained with PCl₅ as theinitiator at −60° C. and 40° C.:

Thus, the formation of a phenolic repeating unit having an amine salttherein occurs preferentially to this kind of monomer(monoparasubstituted benzene ring other than the two sites occupied bythe oxazine ring) at low temperatures (e.g., −60° C.) with a PCl₅initiator. A phenoxy repeating unit occurs more frequently at 40° C.than at −60° C., but still is not the exclusive repeating unit. Theamine salt form of the phenolic repeating unit causes an increased watersolubility. Further, at −60° C. the availability or reactivity of anortho CH position also affects the relative ratio of phenolic to phenoxyrepeat units. Steric hindrance around the ortho CH position or asubstituent in the ortho Cl position favors a phenoxy structure from thebenzoxazine, while the absence of steric hindrance and absence of anortho substituent favors a phenolic structure. A unified theory toexplain the above is that there is a competition between the reactivityof the aromatic carbon (ortho with respect to the oxygen) and the basicnitrogen atom of the (O—CH₂—N(R₂)CH₂). When substitution on the aromaticring is in such a way that it reduces the reactivity of the ring orthocarbon, the protonation or alkylation of the nitrogen dominates and aphenoxy structure results. However, if the ortho CH position (withrespect to the O) is open and no bulky substituent offers sterichindrance, the ring reactivity dominates and a phenolic structure at lowtemperatures is the result. In summary, it is anticipated that at −60°C., if i) aromatic ring reactivity dominates, then a phenolic repeatingstructure is obtained, and if ii) oxazine ring reactivity dominates,then a phenoxy repeating structure is obtained, and that monomers givenbelow will result in an increased probability of the repeatingstructures given.

Monomer Preferred Repeating Structure

Phenolic structure

Phenoxy structure

Phenoxy structure

Mixture phenolic and phenoxy

As phenolic repeat units from benzoxazine are now available from lowertemperature cationic polymerizations, it is desirable to distinguish theresulting polymer from thermally polymerized benzoxazine polymers.Desirably the number average molecular weight of polymers fromcationically polymerized monofunctional benzoxazine monomers are atleast 2,000 or 5,000. Desirably the cationically polymerized benzoxazinewhen both phenolic and phenoxy structures result has at least 2, 5, 10,or 15, or at least 50, 80, or 90 mole % repeating units of the phenoxystructures. However, the pure phenolic structure when the molecularweight is above 2,000 or 5,000 is not excluded.

Five benzoxazine monomers were used for most of the experimentation oncationically initiated polymerization. They were BA-a monomer and C-mmonomer. Different chemical structures for two of the monomers are shownbelow:

BA-a monomer is made from the reaction of bisphenol A, formaldehyde, andaniline. C-m monomer is made from the reaction of cresol, formaldehyde,and methylamine. C-m monomer was prepared according to the procedure setforth by Ning and Ishida in Journal of Polymer Science, ChemistryEdition, vol. 32, page 1121 (1994). The polymerization solvents usedwere 1,2-dichloroethane, 1,2-dichlorobenzene, chloroform, or deuteratedchloroform. All the solvents were dried by conventional methods anddistilled under argon. The cationic initiators used in Table 1 werepurchased from Aldrich Chemical Company and used without furtherpurification. All solvents and reactants were stored in a dry box unlessrefrigeration was required. The moisture was usually less than 1 ppm inthe dry box. Table 1 below shows polymerizations, which were ran forabout 20-80 hours using the specified variety of cationic and otherinitiators. Table 1 sets forth the results of polymerizations of BA-amonomer. While run numbers 8, 9, and 11 did not yield any insolublepolymer, the solutions turned deep red and a tough red polymer wasrecovered when the solvent was evaporated. The initiator amount wastypically about 5 mole percent based upon the amount of benzoxazinemonomer:

TABLE 1 ChCl₃ insoluable as wt. % based Temper- on total Run No.Initiator ature monomer weight 1 Phosphorus pentachloride 20 56.2 (PCl₃)2 Phosphorus trichloride (PCl₃) 20 19.6 3 Phosphorus oxychloride 20 15.0(POCl₃) 4 Titanium (V) chloride (TiCl₅) 19.4 5 Triphenylcarbenium 20 8.7antimonatehexachloride [(C₆H₅)₃C+(SbCl₆)⁻] 6 Antimony pentachloride 5310.1 (SbCl₅) 7 Antimony pentachloride/ 20 12.3 oxetane (SbCl₅) 8 Methyltosylate (MeOTs) — — 9 Methyl triflate (MeOtf) — — 10 Aluminumphthalocynanine 20 20.4 chloride (metallophorphyrin) 11 Triflic acid — —12 Borontrifluoro dietherate 53 0 13 Borontrifluoro dietherate/ 20 0promoter 14 p-tolyl triflate 53 0 15 Methyl iodide covalent 53 0initiator 16 Butyl lithium (anionic) 20 0 17 Benzoyl peroxide (free 110 0 radical)

TABLE 2 Char yield at Initiator Tg by DSC (° C.) 800° C. by TGA (%) PCl₅215 50.26 PCl₃ 216 48.93 POCl₃ 210 50.53 TiCl₅ 222 61.78Metallophorphyrin 186 43.59 MeOTf 193 31.48 MeOTs 142 28.06 Triflic acid— 31.29 (C₆H₅)₃C⁺(SbCl₆)⁻ — — SbCl₅ — — Thermal-cured (Cntrl) 177 28.56

Several of the polymers polymerized according to the condition set forthin Table 1 were analyzed by thermogravimetric analysis (TGA) ordifferential scanning calorimity (DSC). The glass transition temperature(Tg) is used to characterize a particular polymer and microstructure.The char yield is affected by many factors, a high char yield istypically desired for precursors for char or for non-flammable polymers.As can be seen in Table 2, the first four cationic initiators yieldedpolymers from benzoxazine having significantly higher Tg's than the lastsample, which was a control (thermally cured) polybenzoxazine from thesame monomer. The char yield from the polymers initiated with the firstfour cationic initiators varied from about 44 or 49 to about 62 weightpercent based upon the weight of the initial starting polybenzoxazine.This was significantly higher than the control, which yielded only about29 weight percent char under identical conditions. The next threecationic initiators (5-7) yielded polymers with slightly different Tg'sthan the control. The char yield from the fifth through the seventhcationic initiators was also different than that of the control,although the difference from the control amount was not as significantas with the first four cationic initiators. It is to be noted that thepolymer from the methyl tosylate, methyl triflate, and triflic acid hada different color (red) than the polymers from the first four cationicinitiators.

The polymers from the cationic polymerization of benzoxazine monomerBA-a were analyzed by Fourier transform infrared spectroscopy along witha thermally cured polybenzoxazine from the same monomer. These resultsindicated substantially similar infrared spectra when bisphenol A-basedbenzoxazine was used. The polymers compared to the monomer showed thedramatic change in the peaks in the region of 1,000 to 1,350 cm⁻¹,arising from the CH₂ wagging (1327 cm⁻¹ and 1305 cm⁻¹), C—O—C asymmetricstretching (1233 cm⁻¹), C—N—C asymmetric stretching (1159 cm⁻¹), andC—O—C symmetric stretching (1031 cm⁻¹) of the oxazine ring,respectively, as well as the decrease of the peak resolution, indicatethe opening of the oxazine ring and the polymerization of the monomerinto oligomers and polymers.

There are significant spectral differences between the thermallypolymerized polybenzoxazine and the PCl₅ initiated polybenzoxazines. Inthe spectrum of BA-a monomer the peaks centered at 1500 cm⁻¹ and 949cm⁻¹ are major characteristics of the tri-substituted benzene ring inthe benzoxazine structure, corresponding to the in-plane C—C stretchingand the out-of-plane C—H deformation of tri-substituted benzene ring,respectively. These two peaks almost disappear in the thermallypolymerized polybenzoxazine spectrum, meanwhile a new peak centered at1489cm⁻¹, representing tetra-substituted benzene ring mode, appears. Theresults are in accordance with the well-accepted thermal polybenzoxazinestructure shown as below:

However, with some cationic initiators, here PCl₅ has been chosen as anexample, these two peaks at 1500 cm⁻¹ and 949 cm⁻¹ remain unaffectedexcept for the peak broadening, which indicates that in thepolybenzoxazine structure obtained, the majority of the benzene ringsshould still be trisubstituted instead of tetrasubstituted. Further NMRresults are in support of this hypothesis.

Polymers for size exclusion chromatographic (SEC) analysis werecationically polymerized from the BA-a monomer using chloroform as asolvent at about 23° C. using PCl₅ initiator and a mole ratio of 1:100initiator:monomer. Portions of the reactants were withdrawn after 1, 2,2.5, 3.5, 5, and 7 hours of reactions and poured into an excess ofmethylol to cause precipitation. The precipitated material was recoveredand analyzed by SEC for differences in molecular weight as a function ofreaction time. As is well-known in SEC analysis, short retention timesindicate higher molecular weight, while long retention times indicatelower molecular weight. A peak at 29.3 minutes was associated with thebenzoxazine monomer. A new peak occurred between 21 and 29 minutes asthe polymerization occurred. Simultaneously the peak at 29.3 minutesdecreased in size. This is consistent with theories on polymerization.

An NMR experiment was conducted comparing polymer from a cationicpolymerization, using PCl₅ as the initiator in deuterated chloroform asa solvent, to a thermally polymerized polymer. The reaction temperaturefor the cationic polymerization was 40° C., the reactor was a sealed NMRtube, and the material was evaluated after 10 minutes, 30 minutes, 1hour, and 2 hours to see which NMR peaks were being generated ordepleted as the monomer was cationically polymerized. The monomer peaksat 2.6, 3.9, and 4.75 ppm gradually disappeared, while new peaks at 1.95and 3.65 ppm appeared, and a broadening of the peak at 2.2 ppm occurredindicating less molecular rearrangement occurred ongoing from monomer topolymer under cationic conditions. Based on these results (proton NMR)along with carbon ¹³NMR and FTIR results, the repeat structure of thethermally polymerized polymer in these studies is theorized to bepredominantly tetrasubstituted phenolic structure, e.g.,

while the cationically polymerized samples is predominantly the phenoxyrepeating units of the structure

In conclusion, the experimental data indicates the cationicallyinitiated polymerization benzoxazine monomers can occur at substantiallylower temperatures than thermal polymerization and produces polymers ofdifferent microstructure than thermal polymerizations. Further, thesepolymers can have substantially higher char yields 62\29 or an increaseof 200 percent over the char yield of the thermally polymerizedpolymers.

The polymers from benzoxazines are useful as precursors for charyielding material (e.g., precursors to aircraft brake pads). They arealso useful as temperature and flame resistant polymers for electricalcomponents, planes, cars, buses, etc.

A polybenzoxazine material has the following characteristics, making itdesirable as a microelectronic or optoelectronic underfill material:

-   -   (i) Low moisture uptake;    -   (ii) Low volume shrinkage during curing;    -   (iii) Good film-forming properties (i.e., thickness control and        tackiness at room temperature);    -   (iv) A low coefficient of thermal expansion (CTE).

The following table compares the properties of a typical polybenzoxazinecompound with a typical epoxy:

TABLE 3 Typical Polybenzoxazine- Property Based Film Typical EpoxyCuring shrinkage (%) 0 or expansion 3-4 Moisture uptake (%) 1.5  3Modulus (GPa) 4 2-3 Tensile strength (MPa) 130 120 Tg (° C.) 170 150Impact strength (J/m) 30  30 CTE (ppm/° C.) 55  65

Different film structures can be designed to meet applicationrequirements. Film structures according to embodiments of the inventionare shown in FIGS. 1 a-1 e. In each case, a film 2 a-e is providedbetween two removable sheets 1. FIG. 1 a illustrates a film 2 a, whichis a polybenzoxazine-based film as hereinbefore described. FIG. 1 billustrates a polybenzoxazine-based film 2 b includinguniformly-dispersed filler particles 4. FIG. 1 c illustrates apolybenzoxazine-based film 2 c including anisotropic electricallyconductive particles 3. FIG. 1 d illustrates a polybenzoxazine-basedfilm 2 d including electrically conductive columnar components 5. FIG. 1e illustrates a polybenzoxazine-based film having multiple layers 2 ewith components 3 therein. The components 3 typically have a lower CTEthan the polybenzoxazine material of the layers 2 e. More of thecomponents 3 are located in an upper one of the films 2 e than in lowerones of the films 2 e so that the upper film has a lower coefficient ofthermal expansion due to the higher number of components 3 than lowerones of the films 2 e.

FIG. 2 a illustrates an optoelectronic package 8 a and an optoelectronicdie 9 which is attached to a package substrate 6 by means of an adhesivefilm 7. The film 7 may, for example, be the film 2 a illustrated in FIG.1 a. Film 7 shrinks by a relatively small amount during curing so thatthe position of the die 9 is substantially maintained relative to thesubstrate 6. Such a feature is desirable when coupling light fromanother component, which is at a predetermined distance from the packagesubstrate 6, to a feature 9 a on the die 9. A microelectronic packagecan be assembled n a similar manner by replacing the optoelectronic diein 9 with a microelectronic die.

FIG. 2 b illustrates an electronic assembly 8 b having a semiconductordie 13 attached to a package substrate 10 with a film 11 that may be thesame as the film 2 d illustrated in FIG. 1 d. The film 11 thus includesa plurality of columnar members 14. Each member 14 has an upper end incontact with a respective contact 12 on the die 13, and a lower end incontact with a respective pad 15 of the package substrate 10. The die 13is in electric communication with the package substrate 10 through themembers 14. A lower surface of the die 13 adheres to an upper surface ofthe material 11, and an upper surface of the package substrate 10adheres to a lower surface of the material 11. The material 11electrically insulates the members 14 from one another.

FIG. 2 c illustrates an electronic assembly 8 c including asemiconductor die 20 which is attached to a package substrate 16 througha film 18. The die 20 has a plurality of contacts 21 on a lower surfacethereof, and the package substrate 16 has a plurality of pads 24 on anupper surface thereof. The film 18 includes a polybenzoxazine material19 having an upper surface attached to a lower surface of the die 20,and a lower surface attached to an upper surface of the packagesubstrate 16. The film also has columnar members 22 in the material 19.Each member 22 establishes an electrical path between a die and itsrespective substrate, and has a respective upper end contacting arespective one of the contacts 21 and a respective lower end contactinga respective one of the pads 24. The film 18 also has a plurality offiller particles 17 in the material 19. The particles 17 have a lowerCTE than the material 19, and thus decrease the overall CTE of the film18. More of the particles are included near the die 20 than near thepackage substrate 16, so that the CTE of the film 18 is lower near thedie 20 than near the package substrate 16. The die 20 has a lower CTEthan the package substrate 16. The CTE of the film 18 closely matchesthe CTE of the package substrate 16 near the package substrate 16, andclosely matches the CTE of the die 20 near the die 20. A similar packagecan be constructed without columns in the film. It may, for example, bepossible to locate the film of FIG. 1 e on a substrate having bumps orother contacts thereon, or a substrate without bumps but with bumps onthe die. It may also be possible to use the film of FIG. 1 e between adie and a substrate where both die and substrate have facing bumps, sothat the bumps on both the die and the substrate penetrate the film.

Most of polybenzoxazine-based film monomers are solids at roomtemperature. But they normally have low melt viscosity at slightlyelevated temperatures such as 70-80° C. Cured pure polybenzoxazine-basedfilm materials also have a lower CTE than a typical epoxy material,which allows lower filler loading to reach the same CTE value as atypical epoxy. The low melt viscosity and lower filler loadingfacilitate the material flow and wet on contact surfaces, and thereforeprovide a wider process window. Additionally, the lower moisture uptakeof the polybenzoxazine-based film material is expected to have betterperformance of a finished package during moisture testing. There is alsoa slight volume increase during the cationic ring opening polymerizationof the polybenzoxazine material. This volume expansion can effectivelyreduce residual stresses during the cooling of the entire package.

Considering that polybenzoxazine-based film monomers can be easily madethrough Mannich reaction from a phenolic derivative, formaldehyde, and aprimary amine, it can be seen that a polybenzoxazine-based film can besynthesized from a wide selection of raw materials consisting ofphenolic derivatives and primary amines. The molecular structure ofpolybenzoxazine-based films offers superb design flexibility to allowfor the properties of the cured materials to be controlled for thespecific requirements of a wide variety of individual applications.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative and not restrictive of the current invention, andthat this invention is not restricted to the specific constructions andarrangements shown and described since modifications may occur to thoseordinarily skilled in the art.

1. An electronic assembly, comprising: a substrate; a film adhered to anupper surface of the substrate, the film including a polybenzoxazinepolymer and a plurality of columnar members; and an electronic dieadhered to an upper surface of the film, wherein each columnar memberwithin the film electrically connects a respective contact on the diewith a respective pad on the substrate.
 2. The electronic assembly ofclaim 1, wherein the electronic die is one of a semiconductor die and anoptoelectric die.
 3. The electronic assembly of claim 1, wherein thefilm includes particles having a different CTE than the polymer.
 4. Theelectronic assembly of claim 3, wherein the particles are substantiallyuniformly dispersed.
 5. The electronic assembly of claim 3, wherein moreparticles are included in one layer of the film than another layer ofthe film.
 6. The electronic assembly of claim 5, wherein the particlesprovide the film with a different CTE in one layer than in anotherlayer.
 7. The electronic assembly of claim 6, wherein the CTE of thefilm is lower near the die.
 8. The electronic assembly of claim 1,wherein the polymers are derived from the ring opening polymerization ofone or more 2H-1,3,-dihydrobenzoxazine monomers.
 9. The electronicassembly of claim 8, wherein the one or more 2H-1,3-dihydrobenzoxazinemonomers include at least 25 mol % of one or more monomers having theformula

wherein each R₁ individually is one or more groups selected fromhydrogen, alkyls of 1 to 10 carbon atoms, aromatic, alkyl substitutedaromatic, or aromatic substituted alkyl of 6 to 20 carbon atoms, mono orpoly fluorine substituted alkyls of 1 to 20 carbon atoms, mono or polyfluorine substituted compounds having at least one aromatic ring and 6to 20 carbon atoms or a benzoxazine from a phenolic compound of 6 to 20,n is from 1 to 4; wherein each R₂ is an alkyl of 1 to 10 carbon atoms,an aromatic, alkyl substituted aromatic or aromatic substituted alkyl of6 to 20 carbon atoms, or an amine of 1 to 10 carbon atoms, or abenzoxazine of 9 to 20 carbon atoms; and wherein R₃ and p are as definedabove.
 10. The electronic assembly of claim 9, wherein the R₁ group ofthe formula A is ortho to the oxygen of the benzoxazine.
 11. Theelectronic assembly of claim 9, wherein the R₁ group of the formula A ismeta to the oxygen and para to the CH₂—N—(R₂).
 12. The electronicassembly of claim 9, wherein the R₁ group of formula A is para to theoxygen.
 13. The electronic assembly of claim 1, wherein the polymer hasa number average molecular weight of at least 5,000.
 14. The electronicassembly of claim 1, wherein the polymer is the reaction product ofreacting at least one 2H-1,3,-dihydrobenzoxazine monomer with a cationicpolymerization initiator.
 15. The electronic assembly of claim 14,wherein the cationic polymerization initiator is PCl₅.
 16. Theelectronic assembly of claim 14, wherein the 2H-1,3,-dihydrobenzoxa-zinemonomer comprises at least 25 mole percent of one or more monomershaving the formula

wherein R₁ individually is hydrogen, one or more groups selected fromalkyls of 1 to 10 carbon atoms, an aromatic, alkyl substitutedaromatics, or aromatic substituted alkyls of 6 to 20 carbon atoms, monoor poly fluorine substituted alkyls of 1 to 20 carbon atoms, mono orpoly fluorine substituted compounds having at least one aromatic ringand 6 to 20 carbon atoms, or a benzoxazine from a phenolic compound of 6to 20, n is from 1 to 4; wherein each R₂ is an alkyl of 1 to 10 carbonatoms, an aromatic, alkyl substituted aromatic or aromatic substitutedalkyl of 6 to 20 carbon atoms or an amine of 1 to 10 carbon atoms or abenzoxazine of 9 to 20 carbon atoms; and wherein R₃ is H or R₂ and p isan integer between 0 and
 3. 17. The electronic assembly of claim 1,wherein upper ends of the members are coplanar with the upper surface ofthe film.
 18. The electronic assembly of claim 17, wherein lower ends ofthe members are coplanar with a lower surface of the film.
 19. Theelectronic assembly of claim 18, wherein the members are columnar andhave a height being the same as a thickness of the film.